Vitamins and nutrition . Patient guide
Vitamin D (25-OH Vitamin D)
Also known as: 25-hydroxyvitamin D, Calcidiol
Vitamin D is a fat-soluble vitamin made in skin from sunlight and obtained in smaller amounts from food. The blood test for vitamin D status measures 25-hydroxyvitamin D, which reflects total body stores from both routes.
This biomarker entry is being clinically reviewed by our team. The factual content draws on UK guidance (NICE, NHS, Royal Colleges and the relevant speciality society where cited).
Reference range
Reported in nmol/L. Final reports always carry the issuing laboratory's range, which is what your clinician will interpret against.
| Group | Range | Note |
|---|---|---|
| Deficient (NHS classification) | under 25 | nmol/L |
| Insufficient | 25 to 50 | |
| Adequate | 50 to 75 | |
| Optimal (clinical view for symptomatic patients) | 75 to 125 | |
| Above which supplementation should be reviewed | over 220 |
What it is
Skin makes vitamin D3 (cholecalciferol) when exposed to ultraviolet B sunlight. The liver converts it to 25-hydroxyvitamin D, the storage form measured in blood tests. The kidneys then convert it to its active hormonal form, 1,25-dihydroxyvitamin D, when calcium balance demands it.
Why a clinician would order it
Vitamin D status is checked in fatigue, low mood, hair thinning, bone or muscle ache, frequent infections, in patients with darker skin (where UK sunlight is rarely enough), and as part of preventative wellness screening. UK guidance recommends most adults supplement in winter.
If your level is outside the range
Symptoms of low 25-OH Vitamin D
- Persistent fatigue
- Low mood
- Bone or muscle ache
- Hair thinning
- Frequent infections
- Reduced exercise tolerance
What low can indicate. Insufficient sunlight exposure (the commonest UK cause), darker skin, obesity (vitamin D is fat-soluble and less available), malabsorption (coeliac, IBD, gastric surgery), or older age (reduced skin synthesis).
Symptoms of high 25-OH Vitamin D
- Nausea, vomiting (excessive supplementation only)
- Confusion, kidney impairment (severe excess only)
What high can indicate. Excessive supplementation. Rarely, granulomatous disease such as sarcoidosis.
Testing tips
No fasting required. If you take a vitamin D supplement, the result will reflect supplemented status (useful for adjusting dose). UK levels are typically lowest at the end of winter (Feb to April) and highest at the end of summer (Aug to Oct).
Where you can get this tested
Vitamin D is included in the following WMG Health panels. Same-day appointments at our Harley Street clinic, with results clinician-reviewed.
Want a specific combination of markers we do not have a panel for? Build a custom panel and our clinicians will design one for you.
Sources
UK guidance our clinicians use when interpreting this marker.
This page is general patient information, not personal medical advice. A GMC-registered clinician will review your results and tailor any interpretation to you. See our Editorial Policy for how we write and review content.